As is well known, in a process of manufacturing a steel ring-shaped member (such as a raceway ring of a rolling bearing), heat treatment (quench hardening) is carried out on the ring-shaped member so as to impart required mechanical strength and the like to the ring-shaped member. The quench hardening involves a heating step of heating the ring-shaped member to a target temperature (for example, to a temperature range equal to or higher than an A1 transformation point), and a cooling step of cooling the ring-shaped member heated to the target temperature. The heating step can be carried out using, for example, an atmosphere heating furnace such as a mesh belt continuous furnace.
The atmosphere heating furnace has an advantage of being capable of heating a large number of workpieces at the same time. However, the atmosphere heating furnace needs to heat not only a workpiece but also the atmosphere. Hence, the atmosphere heating furnace has a problem of low energy efficiency. In this context, in the heating step, a workpiece may be heated to the target temperature by induction heating (for example, see Patent Literature 1). By the induction heating, a workpiece can be directly heated, thereby being capable of achieving high energy efficiency. Further, when a workpiece to be heated is a ring-shaped member, as described in Patent Literature 1, there may be adopted a method of energizing a heating coil under a state in which a plurality of ring-shaped members retained coaxially with each other are arranged in an opposing region (inner periphery) of the heating coil. In this method, the plurality of ring-shaped members can be inductively heated at the same time, thereby being capable of increasing efficiency of heat treatment.